On a first date or a job interview, you should be a more confident, charming, better put-together, less sweaty version of yourself. Everyone knows that. So here's a less-obvious piece of unsolicited advice: Be well-rested. Because sleep deprivation and social skills don't go together. A new study suggests that sleep deprivation makes it hard for people to pick up signs of happiness or sadness in ambiguous facial expressions. After a night of no sleep, researchers at the University of Arizona found, participants didnt know what to make of understated smiles or questionable frowns.

This study, lead by William D.S. Killgore, adds to a small but growing body of research exploring how sleep deprivation affects the way we process emotional stimuli in the environment. Emotional stimuli are sights and sounds, and other sensory cues, that betray feelings, such as an angry note, news footage of a war scene or any face you come across today. (Here are some examples from studies.)

Past studies have consistently shown that people interpret the same words and faces differently when they're sleep-starved vs. well-rested. But what's been less consistent across studies is how skipping sleep changes perceptions of smiling, smizing, scowling, etc. In one past study, for instance, sleep-deprived participants could recognize angry and happy faces when the faces were exceedingly angry or happy, but not when the emotions were displayed less intensely. In another study, however, sleep loss only appeared to affect participants' ability to identify sad faces. Additionally, a number of studies support the idea that sleep-deprived people are hyper-sensitive to potential threats and thus more likely to interpret neutral faces (e.g., a resting face) as negative.

In the current study, which is actually an analysis of earlier, unpublished findings,* Killgore's team digitally blended images of the same man expressing the "six universal basic emotions": suprise, fear, sadness, happiness, disgust and anger. They morphed each universal emotion with the two other emotions with which it's most frequently confused. So, for instance, they took a "happiness" photo and fused it together with a "surprise" photo to create a spectrum of photos bearing different amounts of both emotions. Here's a sampling of the morphed photos:

The study required 54 participants to take an emotion-recognition test four times, in both sleep-deprived and well-rested states. For the test, participants had to view the morphed-face photos (150 in all) and identify the predominant emotion in each of them. Researchers didn't count the photos that were equal blends of two emotions (e.g., 50 percent happiness/50 percent anger) because either emotion was an equally correct answer.

The current study concerns the results of the second emotion-recognition test, taken after one night of sleep deprivation, when participants had been awake for roughly a day. Researchers found that participants were considerably worse at identifying happiness and sadness, but not the other emotions, after they'd pulled an all-nighter, compared to how they performed after a normal night of rest.

These results lined up with researchers' hypothesis. From a survival standpoint, it makes sense that sleep-deprived people would understand confusing facial expressions of fear, anger and disgust better than confusing expressions of happiness and sadness. Here's why: Sleep deprivation appears to impair emotional regulation, reaction time, focus and physical endurance. As a result, sleep-starved people can't process everything in their environments as quickly or thoroughly as they normally would. So they need to use their limited mental resources for what matters most — staying alive. And, for the sake of survival, it's important to understand signs of potential danger. Faces expressing anger, fear, surprise or disgust send the message that "something's wrong!" Happy and sad faces, on the other hand, don't signal threat; they facilitate bonding.

[pullquote]"All things being equal, a sleep-deprived individual shows a lower threshold for interpreting a face as potentially threatening than when normally rested."[pullquote]

"[Happy and sad faces] essentially communicate that it is acceptable to lower one's defensive posture because it is safe to affiliate or socially appropriate to show empathy," researchers wrote. "During situations when one's cognitive performance is compromised through sleep loss...it may actually confer a survival advantage to misinterpret social/affiliative facial expressions of happiness or sadness as something a bit more ominous."

The findings support the prevailing belief that sleep deprivation affects our perceptions of facial expressions — in different ways for different emotions. "All things being equal," researchers wrote, "a sleep-deprived individual shows a lower threshold for interpreting a face as potentially threatening than when normally rested."

The study offers valuable insight into the relationship between sleep deprivation and emotional processing and builds on existing, related research. For instance, one study on sleep-disorder patients similarly showed that people with insomnia and sleep apnea have trouble identifying happy and sad faces. "This raises the possibility," researchers wrote, "that chronic sleep problems may lead to alterations in emotional processing that are similar to that produced by experimental sleep deprivation."

But, more than anything, the study highlights lingering unknowns and reinforces the need for more research. Future studies should test recognition of facial emotions in a more natural environment, researchers wrote. In real life, we process emotional stimuli in a more dynamic, complex way than the morphed-face experiment reflects — most real-world situations involve multiple, changing faces, amid other sights, sounds and smells competing for our attention.

---------------

*This experiment was part of a larger project that was the basis for a 2008 study by the same researchers. For that study, researchers focused on the results of a test taken after participants had been awake for 61 hours straight and taken prescription stimulants. They performed better on the test in this condition than when they'd taken it after only one night of sleep deprivation. In fact, their scores on the 61-hours-awake test rivaled those from the baseline test, which they'd taken in a well-rested state. So, researchers found, the stimulants seemed to help participants recover some of the emotional-processing skills that had eroded during sleep deprivation.